It is known in the art of position coordinate determining devices, sometimes referred to as digitizers, to provide a construction which employs only a single length of a non-electrical signal propagation medium, i.e., a magnetostrictive medium, for each coordinate dimension and a respective grid comprising a plurality of spaced parallel electrical conductors, each conductor extending transversely from a point adjacent one of the magnetostrictive elements. Timing means are provided to enable measurement of strain wave travel time along each magnetostrictive element between a reference position and a position corresponding to the location of a pointer on the grid.
Such a digitizer is disclosed in U.S. Pat. 4,514,688, which teaches a construction for an automatic coordinate determining device having a tablet with a grid network of first and second sets of parallel spaced electrical conductors, which sets of conductors are mutually orthogonal. Each respective magnetostrictive element (i.e., wire) is situated with its axis transverse to the corresponding set of electrical conductors. A pointer is movably arranged adjacent the conductors. This pointer includes a flux-producing element which is inductively coupled to at least one conductor from each set of parallel conductors. When the flux-producing element is energized, an electrical current is induced in the proximal conductors, which current in turn induces a strain wave in the respective magnetostrictive element at a region adjacent the energized conductor. The magnetostrictive elements correspond respectively to the X and Y coordinate axes. The strain wave induced in the respective magnetostrictive element travels along the axis to a region of the magnetostrictive element where a sensor means is provided. The respective sensor means acts as a sensor for producing a signal in response to detection of the traveling strain wave induced as a result of energization of the pointer.
In addition, in U.S. Patent No. 4,514,688 a fiducial signal induction coil is provided at each end of both magnetostrictive elements. These fiducial signal induction coils are energized to produce strain waves prior to pulsing of the cursor. The fiducial signal induction coils are both energized to produce a pair of traveling strain waves along the magnetostrictive element, which strain waves are received by the sensor means at the end of the magnetostrictive element. The signals output by the sensor means upon detection of the respective strain waves are clocked to determine the time of travel separating the two strain waves. This detected time of travel is then compared with a reference value corresponding to a standard length of the magnetostrictive element. The difference between the detected time of travel and the reference value represents an error which must be compensated for. It should be noted that this calibration process takes place without energization of the flux-producing element. After storage of the value of error compensation required, the logic and control network energizes the flux-producing element of the pointer. The time of travel of the strain wave induced by the pointer is then determined. The latter time of travel for each magnetostrictive element represents the corresponding uncompensated coordinates of the pointer position. Following compensation, the true coordinates of the pointer position are output to data storage or display.
This prior art digitizer has the disadvantage that two magnetostrictive elements are provided for performing twodimensional position coordinate determination. Consequently, additional expense must be incurred in the manufacture of damping mounts and a sensor coil for each magnetostrictive element.
A further disadvantage lies in the fact that if the cursor is located in a corner of the active data tablet surface near where a sensor coil is arranged, pulsing of the cursor will directly induce a pulse in the sensor coil, which is in addition to the pulse induced by the propagating strain wave, unless heavy shielding is provided. In the absence of such shielding, the circuitry would be unable to accurately discriminate between the two pulses, i.e. the informational signal produced by the propagating strain wave and the non-informational signal induced directly by the cursor. This phenomenon is known as overloading.